1. Field of Invention
The present invention relates to a recording medium substrate, a recording medium, a stamper and a mold system for forming recording medium substrate, a substrate for optical disc, an optical disc and a production method for stampers.
2. Description of the Related Art
An optical information recording medium such as a compact disc (CD), a mini-disc (MD), and a digital versatile disc (DVD) is widely in use in audio, video, and computer systems for recording various types of information. However, recording densities of these optical information recording medium need to be further increased as the amount of information that a medium must record is expected to increase significantly.
In order to increase the recording density of recording medium, such as optical discs, it is crucial to reduce the spot diameter of focused laser beam. This requires the use of laser beam with a shorter wavelength or objective lenses with a higher numerical aperture (NA) in the laser optical system.
The laser beam is focused on a data recording surface through a light-transmitting layer formed over the data recording surface. An attempt to decrease the wavelength of the laser beam or increase the NA of the objective lens is likely to cause a significant optical aberration. For this reason, the thickness of the light-transmitting layer must be kept as small as possible so arecordduce the optical aberration.
Production of CD and MD typically involves the use of laser beam with a wavelength of 780 nm, a lens with an NA of 0.45, and a light-transmitting layer with a thickness of 1.2 mm; whereas production of DVD, a recording medium having a higher recording density than CD, requires the use of laser beam with a wavelength of 650 nm, a lens with an NA of 0.6, and a light-transmitting layer with a thickness of 0.6 mm. A substrate formed through the injection-molding process using a transparent material can be used to serve as such a light-transmitting layer.
In the pursuit of higher recording densities, even thinner light-transmitting layers are required as the wavelength of the laser beam is further decreased or the NA increased. However, reduction in the thickness of the substrate while maintaining a high precision has become ever more difficult through the conventional injection-molding process (including injection-compression). In other words, with optical discs of the conventional construction, the attempt to increase the recording density by decreasing the wavelength of laser beam or increasing the NA of the objective lens faces a technical limitation due to the difficulty in making a thinner substrate to serve as the light-transmitting layer.
To overcome such a technical limitation, some optical discs, such as that disclosed in Japanese Patent Laid-Open Publication No. Hei 10-289489, is constructed by first forming a relatively thick protection plate to serve as a substrate. This substrate is formed through injection-molding and does not need to transmit light. A film for data recording/a reflective film for data retrieval is then formed on a data recording surface of the substrate, followed by formation of a thin light-transmitting layer on top of the film. This construction not only permits formation of thin light-transmitting layers but also provides mechanical strength for the overall structure. Thus, such optical discs can readily be adapted to laser beam with a decreased wavelength or a lens with an increased NA.
The light-transmitting layer may be formed by spin-coating a UV-curing resin solution onto a substrate and curing the resin, or by placing a separately prepared transparent sheet over the substrate and applying an adhesive such as a UV-curing resin or an adhesive material to bind the sheet to the substrate. However, the approach using the transparent sheet to be bound to the substrate has strict requirements for the optical characteristics and the precision in thickness of the light-transmitting layer and thus leads to a high manufacturing cost. In comparison, the approach using the spin-coating process can achieve the required characteristics relatively easily. Specific examples of the spin-coating processes used to form light-transmitting layers are disclosed, for example, in Japanese Patent Laid-Open Publication No. Hei 10-289489, Japanese Patent Laid-Open Publication No. Hei 11-73691 and Japanese Patent Laid-Open Publication No. Hei 11-203724.
Although the recording density can be increased through the use of the optical discs with the above-described structure, which includes a protection plate to serve as a substrate, these optical discs have problems in terms of their manufacturing technologies. These problems are as described below:
1) Problem of Errors in Substrate Contruction
When injection-molded, the substrate may have a larger thickness in the area near its periphery than in the area near its center because different areas of a substrate cool down at different rates. Specifically, as shown in FIG. 12, a bump 2, called as xe2x80x9cski jumpxe2x80x9d, may be formed on the periphery of an injection-molded plastic substrate 1. The bump 2 may affect the incident angle and the reflection angle of the laser beam or it may collide with the laser optical system such as an objective lens. For this reason, the substrate must be post-treated to remove the bump 2 as described in, for example, Japanese Patent Laid-Open Publication No. 5-200791. This leads to an increased manufacturing cost.
2) Problem of Flash Formed on Substrates
Referring to FIG. 13, a mold system 100 is shown for manufacturing substrates of the type described above. The mold system 100 includes a ring mold 102 to form an outer peripheral surface of a substrate 1, planar molds 104 and 106 arranged on opposite sides of the ring mold 102 to form top and bottom surfaces of the substrate 1, respectively, and a pair of stampers 108 placed adjacent to the planar molds 104 and 106. Only one stamper 108 may be placed adjacent to one of the planar molds 104 and 106 although two of them are arranged in the example shown. When only one stamper 108 is arranged adjacent to either one of the planar molds 104 and 106, the other of the planar molds 104 and 106 may be formed integrally with the ring mold 102.
The stamper 108 includes a transfer surface 108A on which microscopic projections/recesses are formed to correspond to the information signals and information grooves to be record. The substrate 1 with a surface having the microscopic projections/recesses transferred from the transfer surface 108A of the stamper 108 can be obtained by filling a cavity defined by the mold system 100 with a resin material. The area including the transferred microscopic projections/recesses serves as a data recording region.
However, as shown by an enlarged view in FIG. 14(A), the resin material tends to seep into the gap between the ring mold 108 and the stamper, forming a flash B on the circumference of the substrate 1. As shown in FIG. 14(B), when the substrate 1 is removed from the mold system 100, the flash B may be bent in a direction perpendicular to the surface of the substrate to project from the data recording region (i.e., the surface of the substrate) or it may come off the substrate 1 and stick to the data recording region. If the substrate 1 with such flash B is subjected to spin-coating to form a light-transmitting layer, the flash B causes accumulation of the coating solution. This results in non-uniform, or defective film formation. For this reason, the flash B must be cut off or removed in advance. This is also the case with the conventional optical recording medium, which employ the substrate to serve as a light-transmitting layer.
3) Problem of Damage to Molds
As described above with reference to FIG. 14, the stamper 108 and the ring mold 102 need to be pushed against one another by applying a substantial force in order to eliminate the gap and thereby reduce the occurrence of the flash B. This can damage edges 110A of inner peripheral wall 10 of the ring mold 102 and lead to frequent replacement of the ring mold 102. Furthermore, thermal expansion or contraction of the stamper caused by the heat of the molten resin is disrupted, making the molding process difficult.
4) Problem of Non-Uniform Thickness of Light-Transmitting Layer
The spin-coating process is a process to apply a coating solution to a surface of a substrate by letting the coating solution flow over the surface while the substrate is rotated on a spindle. Rotation of the substrate causes the coating solution to evenly spread over the surface of the substrate. Although the process is advantageous in that the thickness of the coating solution may be adjusted by controlling the rotation rate of the spindle, the time that the coating is applied, and the viscosity of the coating solution, it is accompanied by a problem that the coating solution tends to accumulate in the area near the circumference of the substrate, forming a thick part there. This problem is further described below.
Referring to FIG. 15, for example, the substrate 1 includes on its top surface a blank region 3 having a predetermined width and extending on an outermost part of the substrate 1, and a signal recording region 4 extending adjacent to the blank region 3 on the inner side of the blank region 3. A light-transmitting layer 5 is arranged over the top surfaces of the blank region 3 and the signal recording region 4. The light-transmitting layer 5 is formed by spin-coating a resin material and curing it, such that a bump 5A is formed over the outermost part of the signal recording region 4 and the blank region 3. If the light-transmitting layer 5 has a relatively large thickness (for example, 100 cm) for the layers formed by using the spin-coating process, the thickness T and the width W of the bump 5A become so large that the bump 5A overlaps the signal recording region 4 and thereby reduces the area that can be used for recording information.
Aside from this, there is a significant chance that the bump 5A collides with the laser optical system. Accordingly, the substrate must be subjected to a post-treatment to remove the bump 5A as described, for example, in Japanese Patent Laid-Open Publication No. Hei 11-86355 and Japanese Patent Laid-Open Publication No. Hei 11-86356. This leads to an increased manufacturing cost.
Japanese Patent Laid-Open Publication No. Hei 11-86356 describes a process for removing the bump 5A as shown in FIGS. 16(A) and 16(B), in which a light-transmitting layer 5 is first formed on a surface of a substrate 1 having a larger diameter than that shown in FIG. 15, and the entire outer portion of the substrate 1 including the bump 5A is subsequently removed by, for example, cutting.
A drawback of this approach is that, if the removal of the outer portion of the substrate is not carried out in a precise manner, the concentricity of the outer peripheral surface (circumferential side) of the substrate with respect to the inner peripheral surface of the substrate can be decreased, affecting the dynamic balance of the disc when it is mounted on a disc drive and rotated. As a result, normal recording/playing operations of the disc are interrupted. In addition, the process has a low yield in terms of the materials of the substrate since the outer portions of the substrates are discarded.
The present invention addresses the above-mentioned problems associated with the prior art. Accordingly, it is an objective of the present invention to reduce the manufacturing cost of the recording medium substrates and prevent the non-uniform area on the outer periphery of the substrates or the light-transmitting layers from expanding, without reducing the concentricity and without requiring a significant cost. It is also an objective of the present invention to provide an optical disc formed from the recording medium substrate in which non-uniform area on the outer periphery of the light-transmitting layer is prevented from expanding.
In the course of his studies on the structure of stampers and molds, and the shape and the structure of substrates, the present inventor has discovered that the non-uniformity in the outer periphery of the light-transmitting layer can be prevented by modifying the shape of the substrate. In summary, the above-described objectives are achieved by the following aspects of the present invention.
(1) A stamper for forming at least a data recording region on a recording medium substrate through a transfer surface thereof, the transfer surface transferring a predetermined set of data to the recording medium substrate, the stamper fitted on at least one of metal molds that are arranged opposite one another for forming the recording medium, wherein a stepped portion formed as bends in the material of the stamper, and the stepped portion is configured to project from the transfer surface toward the recording medium substrate.
(2) The stamper described in (1) above, wherein the stepped portion is formed to have a circular shape and the circle is sized so that an end surface of the stepped portion engages an edge of an inner peripheral wall of the mold, the inner peripheral wall forming an outer peripheral surface of the recording medium substrate.
(3) The stamper described in (2) above, wherein the end surface of the stepped portion is bent on either radial end thereof so that the stepped portion has a radial cross-section that projects toward the recording medium substrate.
(4) A mold system including: a ring mold having an inner peripheral wall for forming an outer peripheral surface of a recording medium substrate; a pair of planar molds arranged opposite one another for forming top and bottom surfaces of the recording medium substrate; and a stamper fitted on at least one of the pair of planar molds, the stamper forming a data recording region on the recording medium substrate through a transfer surface thereof, the transfer surface transferring a predetermined set of data to the recording medium substrate; wherein the stamper is provided with a stepped portion formed as bends in the material of the stamper, the stepped portion being configured to project from the transfer surface toward the recording medium substrate.
(5) The mold system described in (4) above, wherein the stamper is fitted on the planar mold with a side of the stepped portion that is opposite to the substrate defining a space.
(6) A recording medium substrate made by using a mold system, the mold system including: a ring mold having an inner peripheral wall for forming an outer peripheral surface of a recording medium substrate; a pair of planar molds arranged opposite one another for forming top and bottom surfaces of the recording medium substrate; a stamper fitted on at least one of the pair of planar molds, the stamper forming a data recording region on the recording medium substrate through a transfer surface thereof, the transfer surface transferring a predetermined set of data to the recording medium substrate, wherein the stamper is provided with a stepped portion formed as bends in a material of the stamper, the stepped portion being configured to project from the transfer surface toward the recording medium substrate.
(7) The recording medium substrate according to (6), wherein the recording medium substrate has a circumferential recess formed radially outside the data recording region on at least one of the surfaces of the recording medium substrate, the recess formed by the stepped portion of the stamper in the mold system.
(8) The recording medium substrate according to (7), wherein a radially outermost edge of the recess on one of the surfaces is lower than a radially innermost edge of the recess as viewed along the thickness of the recording medium substrate.
(9) The recording medium substrate according to any one of (7) or (8), wherein the radially outermost edge of the recess on one of the surfaces is lower than the radially innermost edge of the recess and is higher than a deepest point of the recess as viewed in the thickness of the recording medium substrate.
(10) The recording medium substrate according to any one of (7) to (9), wherein the recess has a depth that gradually decreases as it extends radially outward from the deepest point, the recess substantially smoothly continues to a peripheral region thereof so that the peripheral region has saw-tooth shape in its radial cross-section.
(11) The recording medium substrate according to any one of (7) to (10), wherein the radially innermost edge of the recess is positioned at a distance of 0.965d from the center of the substrate or further, preferably 0.975d or further, and more preferably 0.983d or further, assuming d is the radius of the outermost circumference of the substrate.
(12) A recording medium including a recording medium substrate and a light-transmitting layer disposed on at least one of surfaces of the recording medium substrate, wherein the recording medium substrate is made by using a mold system, the mold system including: a ring mold having an inner peripheral wall for forming an outer peripheral surface of a recording medium substrate; a pair of planar molds arranged opposite one another for forming top and bottom surfaces of the recording medium substrate; a stamper fitted on at least one of the pair of planar molds, the stamper forming a data recording region on the recording medium substrate through a transfer surface thereof, the transfer surface transferring a predetermined set of data to the recording medium substrate, wherein the stamper includes a stepped portion formed as bends in a material of the stamper, the stepped portion being configured to project from the transfer surface toward the recording medium substrate.
(13) A method for producing a stamper using a stamper production disc having a negative pattern region that includes microscopic projections/recesses that are a negative image of a data pattern to be formed on a recording medium substrate, the stamper being fitted on at least one of metal molds that are arranged opposite one another for forming the recording medium substrate, the stamper forming a data recording region on the recording medium substrate through a transfer surface thereof, the transfer surface transferring a predetermined set of data to the recording medium substrate, the method comprising the steps of: forming on the stamper production disc a circumferential recess for forming a stepped portion, the recess having a larger depth and a larger width than the microscopic projections/recesses; and forming the stamper by using the stamper production disc, the stamper having the circumferential stepped portion that is formed by bending a material of the stamper by means of the recess, the stamper having a transfer surface transferred from the negative pattern region.
(14) A method for producing a stamper using a master disc and a mother disc, the stamper being fitted on at least one of metal molds that are arranged opposite one another for forming a recording medium substrate, the stamper forming a data recording region on the recording medium substrate through a transfer surface thereof, the transfer surface transferring a predetermined set of data to the recording medium substrate, the method comprising the steps of: forming the master disc having a negative pattern region that is a negative image of a data pattern to be formed on the recording medium substrate; forming the mother disc using the master disc, the mother disc having a positive pattern region transferred from the negative pattern region; forming on the mother disc in the vicinity of the positive pattern region a recess for forming a stepped portion, the recess having a larger depth and a larger width than microscopic projections/recesses of the positive pattern region; and forming the stamper using the mother disc, the stamper having the stepped portion formed by the recess as bends in a material of the stamper, the stamper having the transfer surface transferred from the positive pattern region.
(15) A method for producing a stamper using a master disc and a mother disc, the stamper being fitted on at least one of metal molds that are arranged opposite one another for forming a recording medium substrate, the stamper forming a data recording region on the recording medium substrate through a transfer surface thereof, the transfer surface transferring a predetermined set of data to the recording medium substrate, the method comprising the steps of: forming the master disc having a positive pattern region that substantially matches a data pattern to be formed on the recording medium substrate; forming a first mother disc using the master disc, the first mother disc having a negative pattern region transferred from the positive pattern region; forming a second mother disc using the first mother disc, the second mother disc having a second positive pattern region transferred from the negative pattern region; forming a recess for forming a stepped portion either on the master disc in the vicinity of the positive pattern region or on the second mother disc in the vicinity of the second positive pattern region, the recess having a larger depth and a larger width than microscopic projections/recesses of the patterns; and forming the stamper using the master disc or the second mother disc, the stamper having the stepped portion formed by the recess as bends in a material of the stamper, the stamper having the transfer surface transferred from the positive patter region or the second positive pattern region.
As used herein, a xe2x80x9csignal recording regionxe2x80x9d refers to a region in which signals are pre-recorded, or to be recorded, in a substantially continuous manner, starting from the inner side of the substrate. As used herein, a xe2x80x9ccenter of the substratexe2x80x9d refers to a point given by an intersection of at least two imaginary lines each representing a diameter of the substrate.
The present inventor has taken advantage of a conventional stamper, whose primary purpose is to form (or transfer) the data recording region on the substrate, and modified it to actively control and determine the shape of the substrate. Unlike the microscopic projections/recesses, which are formed on the surface of the stamper and intended for recording information in the forms of pits and grooves, the above-described stepped portion formed on the stamper serves to shape the substrate and is formed as a xe2x80x9cbendxe2x80x9d in the stamper. As used herein, a xe2x80x9cbendxe2x80x9d refers not only to constructions obtained by bending a planar stamper through processes such as press working but also to those pre-formed into the stamper upon its formation through processes such as electroplating to include a seemingly bent portion.
In this manner, any recessed shape can be integrally formed into the substrate by making use of the stepped portion of the stamper while positively minimizing formation of flash.
In particular, a circular recess can be formed on the outer periphery of the substrate by employing a circular stepped portion. This prevents formation of a so called ski jump on the substrate.
Although formation of flash is inevitable at the circumferential edge of the substrate, it is formed on the substrate along a radially outermost edge of the recess, where the end surface of the stepped portion engages the inner peripheral wall of the mold. Since the position of this flash is lower than that of the data recording region of the substrate when viewed along the thickness of the substrate, accumulation of the coating solution during formation of the reflective layer or the light-transmitting layer through spin-coating is prevented. Otherwise, the accumulation of the coating solution affects the data recording region.
Although formation of the recess on the optical disc substrate appears to be irrational as it seemingly prevents the coating solution from smoothly spreading out during spin-coating, in reality, the centrifugal force resulting from rotation of the substrate causes the coating solution to spread past the recess toward the peripheral region (or, the radially outermost edge of the recess), forming a bump there, and when the rotation of the substrate is stopped, the solution forming the bump flows back into the recess.
As a result, formation of the coating solution bump is effectively prevented. In addition, the radial width of the recess can be made relatively small while preventing bump formation, making it possible to design a larger data recording region. The recess of such construction also serves to prevent increase in thickness of the peripheral region of the substrate caused by thermal contraction that occurs after injection-molding and thus serves to prevent formation of the so called ski jump.
In short, the recess formed by the stepped portion in the peripheral region of the substrate helps eliminate the coating solution bump formed in the peripheral region during the spin-coating process. Hence, any layer, including the light-transmitting layer, can be disposed on the substrate with a smoother and flatter surface than ever.
Further, by making use of the recess and forming the peripheral region of the substrate to have a saw-tooth shape (in its cross-section), the coating solution can readily spatter off the substrate during spin-coating and accumulation of the coating solution in the bump is reduced. This further reduces bump formation.
As set forth, the present invention, employing a simple construction of a recess formed on the substrate, effectively prevents thickening of the substrate in its peripheral region as well as formation of a bump in the periphery of the light-transmitting layer and thereby realizes a significant reduction in manufacturing costs.